The invention relates to torque-limiting screwdrivers which cease imparting a rotary force when a predetermined torque is reached.
In many mechanical operations it is desirable to tighten a screw with a well defined torque, such as when fastening brittle or otherwise easily damaged items, and it is desired to have as high a fastening force without exceeding a predetermined limit. One example of such brittle items is a carbide insert in a turning or milling tool which is secured by a fastening screw. Another example is when a screw is to be threaded into a material having limited strength such as aluminum castings.
For such operations it has long been known to use screwdrivers or other tools which let the handle rotate freely without rotating the screw-engaging tool shank when a preset torque is reached. Examples of hand-held tools of such types are shown in U.S. Pat. Nos. 2,396,040, 2,797,564, 3,890,859, 4,063,474, 4,517,865; British Patent 900,035 and German Patent 40 22 763.
Some of the requirements in connection with torque-limiting screwdrivers are that (a) the torque limit in the tightening direction should be adjustable and defined with high precision, i.e., better than 10% error independent of moisture, wear and lubrication, (b) the torque in the loosening direction should be unlimited, and (c) the act of reaching the limit torque should not cause axial impacts in the tool shank. All known prior art designs fail in one or more of those requirements.
Precision in determination of the torque limit is difficult to achieve with metal release mechanisms, since the friction in such mechanisms is strongly dependent on wear and lubrication. This is true for the following clutch type arrangements: (a) two multi-toothed washers in mutual contact (U.S. Pat. No. 4,517,865), (b) a toothed washer in contact with a single tooth (U.S. Pat. No. 2,797,564), and (c) a toothed washer in contact with a cylinder (British Patent 900,035).
To lessen the dependence upon friction, it has been suggested to provide a clutch in the form of a toothed washer in contact with rollers or balls (see U.S. Pat. No. 3,890,859), but that is difficult to combine with unlimited torque in the loosening direction unless there is added a separate ratchet mechanism such as the type disclosed in U.S. Pat. No. 2,396,040. Tools employing leaf springs acting against profiled shafts or tubes as shown in German Document 40 22 763 or U.S. Pat. No. 5,746,298 are very dependent on friction, and have low precision, e.g., around 20%.
All of the above-mentioned designs will also cause harmful axial impacts and vibrations in the screwdriver shank when the limit torque is reached, except those employing profiled shafts or tubes.
The present invention concerns a screwdriver where the mechanism and special choice of materials permit definition of a torque limit with high precision, and which is made from considerably fewer components than prior art designs, and which avoids essentially all axial impacts or vibrations in the screwdriver shank.
The invention pertains to a torque-limiting screwdriver which includes a screw-engaging shank disposed in a handgrip, and a torque-limiting element for transmitting a screw-tightening torque between the handgrip and the shank preventing the torque from exceeding a predetermined value. The shank includes a rear end engaging the handgrip for transmitting axial force from the handgrip to the shank. The shank includes teeth spaced from the rear end. A spring is arranged to yieldably urge the torque-limiting element into rotation-transmitting relationship with the teeth.